As you move to the left in the NMR spectrum, the protons become more deshielded. So why is it that they require MORE energy to change their spin? If they don't require more energy, then why is the left side labeled as high frequency? Thanks.
The effect of the chemical shielding is to reduce the effective magnetic field at the nucleus. A more deshielded nucleus will have a greater splitting between the different spin orientations, since the splitting is directly proportional to the magnetic field. This leads to a greater transition frequency.